Glass material

ABSTRACT

A glass including SiO 2 , Na 2 O, MgO, Al 2 O 3  and cobalt oxide, wherein the cobalt oxide is 4.5-85 wt % as an oxide of CoO or 4.9-91 wt % as an oxide of Co 3 O 4 .

CROSS REFERENCE TO RELATED APPLICATION

This is a Continuation of U.S. application Ser. No. 09/432,782, filedNov. 3, 1999 now U.S. Pat. No. 6,177,169 issued Jan. 23, 2001, which isa continuation of U.S. application Ser. No. 09/090,382, filed Jun. 4,1998 now U.S. Pat. No. 5,985,401 issued Nov. 16, 1999, the subjectmatter of which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates to an information recording medium, andmore particularly, to an optical information recording medium, which iscapable of reading out or recording with a high recording density, andwhich has a high reliability in repeating recording and regenerationoperations.

Conventionally, compact disks (CD), laser disks (LD), and the like areused widely as optical information recording media. Currently, a DVD,which has seven times the recording density of a CD, has come intopractical use. The DVD is being developed as an erasablerecording-regenerating medium in addition to a read only medium(DVD-ROM), wherein information is directly written onto the substrate.Furthermore, the practical use of a DVD as a RAM for a computerpresently is under investigation.

With the DVD, high density recording can be achieved by using a laserhaving a shorter wave length, such as 650 nm, than the laser used for aCD (wave length approximately 780 nm). However, in order to handle alarge amount of information, such as computer graphics and the like, itis necessary to achieve a higher recording density, such as 1.5 to 2times that of the conventional high density recording. In order toachieve such a high recording density, a semiconductor laser of green toblue color having a shorter wave length (wave length 520-410 nm) thanever is under development.

As another means to achieve a higher recording density, a superresolution film can be employed. The super resolution film is a thinfilm formed at a lower plane of the recording medium, with which a highrecording density can be achieved by the fact that it is able todecrease the size of the beam spot of the incident light passing throughthe film.

One of the mechanisms of the super resolution effect is anabsorption-saturation phenomenon, which is a phenomenon utilizingnon-linear optical characteristics of the super resolution film suchthat the film allows light having a larger intensity than the amount ofits absorption-saturation to pass through the film and absorbs any lighthaving an intensity less than the amount of its absorption-saturation.The spatial intensity of a laser beam utilized in reading and writinghas a Gaussian distribution. Therefore, when the laser light beam passesthrough the super resolution film, the laser light in the lower endportion of the Gaussian distribution, where the intensity is low, isabsorbed by the film, and the laser light in the middle portion of theGaussian distribution, where the intensity is high, passes through thefilm. Accordingly, the diameter of the laser beam is reduced as itpasses through the super resolution film.

An organic thin film made of a material in the phthalocyanine group, asdisclosed in JP-A-8-96412 (1996), chalcogenide, fine particles of acompound semiconductor, and the like are known at the present asmaterials which may be used for the super resolution film describedabove. Additionally, trials to use some organic materials, such asthermochromic materials of the type disclosed in JP-A-6-162564 (1994),and photochromic materials of the type disclosed in JP-A-6-267078(1994), as the super resolution film have been carried out.

However, the above-mentioned materials have problems in reliability andproductivity. That is, there has been a concern about gradualdeterioration of the organic thin film after repeated recording andregenerating operations, because the energy density of a laser beam islocally increased significantly during the recording and regeneratingoperations. Therefore, a sufficient guarantee period for the recordingand regenerating operations is scarcely obtained under a severecondition of use, wherein the recording and regenerating operations areperformed frequently, such as when the disk is used as a RAM and thelike for computers.

On the other hand, chalcogenide is chemically unstable, and so a longguarantee period can not be obtained for this material, and the fineparticles of a compound semiconductor provide difficulties during theproduction process.

SUMMARY OF THE INVENTION

One of the objects of the present invention is to provide an opticalrecording medium having a super resolution film, which can guaranteerepeated recording and regenerating operations for a sufficiently longtime, and which has a preferable productivity and a high resolutioneffect.

A first aspect of the present invention to solve the above issues is anoptical information recording medium comprising a substrate, whereon arecording layer for recording information is formed; and a glass thinfilm, formed onto the substrate, having a characteristics such that theintensity distributions of irradiated light onto the glass andtransmitted light through the glass vary in a non-linear manner.

The substrate is desirably transparent to light, and for instance, ismade of inorganic materials, such as glass and the like, and organicmaterials, such as polycarbonate, polyethylene terephthalate, and thelike are also desirable. Here, the term glass refers to amorphous solidoxides and general amorphous materials containing the above oxide as amain component.

Forming on a substrate includes both forming onto the surface of asubstrate directly and forming onto the surface of a substrateindirectly via another layer, for instance, a protection layer.

In accordance with the above composition, an information recording disk,which has a large capacity, and which experiences less deteriorationafter repeated reading out and writing, can be provided.

In the first aspect of the invention, the recording layer can beprovided with a pit pattern representing the recording information. Thepit pattern is a device by which the information is recorded inaccordance with the arrangement of pits provided onto the surface of thesubstrate. If this recording method is employed, the recordedinformation can not be rewritten. However, once a master die of thesubstrate having this recorded information is made, a large number ofsubstrates with the same information can be manufactured readily.Therefore, this recording method is used for recording movies, music,and computer programs.

The recording layer of the invention can also be a device for recordinginformation with optical energy. For recording information with opticalenergy, an information recording substrate using so-called phasechanging organic materials or inorganic materials, the crystallinestructure of which varies when irradiated by light, is used as therecording layer.

A second aspect of the present invention is an optical informationrecording medium comprising at least a substrate, a recording layer forrecording information formed o n the substrate, and a reflecting filmfor reflecting light formed on the recording layer, wherein thesubstrate is made of glass, the optical transmittance of which increasesin a non-linear manner corresponding to a n increase in intensity of theirradiated light.

In accordance with the above composition, a reflection type informationrecording disk, which has a large capacity and less deterioration afterrepeated reading out and writing, can be provided.

This second aspect of the invention provides an information recordingsubstrate of a type, which reflects incident light with a reflectingfilm provided at a lower portion of the recording film, and reads theinformation with reflected light.

The glass in the first or the second aspects of the invention desirablycontains at least an element selected from transition metal elements andrare earth metal elements.

For the above transition metal elements and the rare earth metalelements, particularly, at least an element selected from the groupconsisting of Tiy, V, Cr, Mn, Fe, Co, Ni, Nd, Ce, Pr, Sm, Eu, Tb, Ho,Er, and Tm is desirable.

When the transition metal element or the rare earth metal element formsa glass film, the metal element is desirably contained in the range from20% by weight to 90% by weight as an oxide to the total weight of theglass. When the metal element forms a glass substrate, the metal elementis desirably contained in the range from 0.1% by weight to 29% by weightas an oxide to the total weight of the glass.

In the first aspect of the invention, the glass desirably contains asoxide the following compounds: SiO₂: 6-80% by weight, R₂O: 0-20% byweight ®=alkali metal element), B₂O₃: 0-30% by weight, and CoO: 20-90%by weight.

In the second aspect of the invention, the glass desirably containscobalt oxide as CoO in the range of 0.1-29% by weight.

A third aspect of the present invention is an information recordingmedium comprising at least a substrate, whereon a recording layer forrecording information is formed, and a super resolution layer formed onthe substrate, the optical transmittance of which increases in anon-linear manner corresponding to an increase in the intensity of theirradiated light, wherein an output maintaining rate of the informationrecording medium after repeating the recording by 10⁴ times is at least90%.

The output maintaining rate is a value indicating how much of theintensity of the electrical signal is maintained after repeating therecording and regeneration by 10⁴ times, taking the intensity of theelectrical signal at the first regeneration of information afterperforming the first recording with irradiation of light as 100%. If thesuper resolution film is deteriorated by repeating the irradiation oflight, the spot size of the laser ray which reaches the recording layeris expanded, and, as a result, the electric output is decreased. Thatmeans that a super resolution film which can maintain the initial outputmaintaining rate as long as possible is desirable.

Furthermore, in accordance with a fourth aspect of the presentinvention, an information recording medium is provided, which comprisesa transparent substrate, and a recording layer for recording informationwhich is formed onto the substrate, wherein an output decrease inrecorded signal at a frequency of 8 MHz is less than −30 dB of theoutput at 1 kHz, and an output maintaining rate after repeating therecording by 10⁴ times is at least 90%.

FIG. 8 is a graph indicating a relationship between the recordingfrequency and the output for the information recording media with andwithout the super resolution film of the present invention. The mediumwith the super resolution film can record signals of higher frequencycomponents, because the spot size of the laser beam reaching therecording layer is decreased. The above composition indicates an indexwhich represents how high a frequency component can be recorded.

In accordance with a fifth aspect of the present invention, glasscomprising SiO₂: 6-80% by weight, R₂O: 0-20% by weight ®=an alkali metalelement), B₂O₃: 0-30% by weight, CoO: 20-90% by weight, as equivalentoxide, respectively, is provided.

The above glass can be mounted not only on a photo disk, but also onvarious media, as a film having the super resolution effect. Forinstance, a display apparatus, which generates light when itsfluorescent body is irradiated with a laser ray so as to be excited, canproduce a high resolution display by mounting the grass film of thepresent invention onto a surface of the fluorescent body, because thespot size of the laser ray can be converged.

In accordance with a sixth aspect of the present invention, a glass thinfilm containing cobalt oxide in the range of 20-90% by weight asequivalent CoO is provided.

In the case of this glass film, the upper limit of the CoO content isrestricted, because, if CoO is added excessively, the CoO isprecipitated, and causes devitrification.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further objects and novel feature of the present inventionwill more fully appear from the following detailed description when thesame is read in connection with the accompanying drawings. It is to beexpressly understood, however, that the drawings are for purpose ofillustration only and are not intended as a definition of the limits ofthe invention.

FIG. 1 is a schematic cross section of a RAM disk according to thepresent invention;

FIG. 2 is a schematic cross section of a simulated sample according tothe present invention;

FIG. 3 is a graph indicating a dependency of transmittance on wavelength of the glass thin film according to the present invention;

FIG. 4 is a diagram showing an XPS of Co of a glass thin film accordingto the present invention;

FIG. 5 is a graph indicating a relationship between transmittance forlight of 650 nm and CoO content;

FIG. 6 is a diagram showing a SIMS of a glass thin film formed onto aglass substrate having a target composition;

FIG. 7 is a schematic cross section of a ROM disk according to thepresent invention;

FIG. 8 is a graph indicating a reading out frequency dependence of anoutput obtained from the ROM disk shown in FIG. 7;

FIG. 9 is a graph indicating a relationship between mark length andvariation in output obtained from the RAM disk shown in FIG. 1;

FIG. 10 is a graph indicating a dependency of output on the repeating ofoperations on the RAM disk shown in FIG. 1;

FIG. 11 is a graph indicating a relationship between CoO content andvariation in reading out of the output obtained from the RAM disk shownin FIG. 1;

FIG. 12 is a schematic cross section of a RAM disk according to thepresent invention;

FIG. 13 is a schematic cross section of a ROM disk according to thepresent invention;

FIG. 14 is a graph indicating variations of laser beam diameter when theglass film of the present invention is formed and not formed; and

FIG. 15 is a block diagram of an apparatus using the photo disk of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

Details of the present invention will be explained hereinafter withreference to various preferred embodiments.

The composition of a number of glass targets investigated in thedevelopment of the present invention is indicated in Table 1.

TABLE 1 Composition (% by weight) Target No. SiO₂ Na₂O CaO MgO Al₂O₃Co₃O₄ CoO (kind) Film Q¹⁾ 1 70.4 13.6 7.8 4.0 1.3 2.9 — glass ◯ 2 69.013.3 7.6 3.9 1.3 2.9 — glass ◯ 3 51.8 10.0 5.7 2.9 1.0 28.6 — glass ◯ 445.3 8.8 5.0 2.6 0.9 37.4 sint²⁾ ◯ 5 29.0 5.6 3.2 1.6 0.6 60.0 sint ◯ 614.5 2.8 1.6 0.8 0.3 80.0 sint ◯ 7 5.9 1.4 0.5 0.2 0.1 91.9 sint X 8 — —— — — 100 sint X 9 51.8 10.0 5.7 2.9 1.0 — 28.6 glass ◯ 10  29.0 5.6 3.21.6 0.6 — 60.0 sint ◯ 11  14.5 2.8 1.6 0.8 0.3 — 80.0 sint ◯ 12  5.9 1.40.5 0.2 0.1 — 91.9 sint X 13  — — — — — — 100 sint X Remarks: ¹⁾ Filmquality ²⁾ sintered target

In Table 1, the column indicating the film quality was provided with Owhen a uniform film was obtained in view of transparency, uniformity,and the like, and with X when the obtained film was not uniform.

In the present embodiment, a soda-lime group glass was used as a motherglass, and a cobalt oxide, which had a large absorption in the vicinityof 650 nm, was used as the transition metal. As raw materials for thecobalt oxide, CO₃O₄ and CoO were used.

The targets No. 1-No. 7 are composed of soda-lime glass and CO₃O₄. Amongthem, targets No. 1-No. 3 were targets in the form of a glass block,because they were vitrified. Targets No. 4 No. 7 were not vitrified,because the content of CO₃O₄ was too much to be vitrified. Therefore, asintered body of a mixture of glass powder and CO₃O₄ was prepared as asintered target.

Target No. 8 is a comparative example of a sintered target made of onlyCO₃O₄.

In targets No. 9-No. 13, CoO was used as the raw material for cobaltoxide. In these cases, target No. 9 was a glass target, because targetNo. 9 had a Co content of 28.6% by weight and was vitrified. Becausetargets No. 10-No. 12 were not vitrified, a sintered target of themother glass raw material and CoO was used.

Target No. 13 is a comparative example of a sintered target made of onlyCoO.

The glass block for the target was obtained by the steps of weighing adesignated amount of powdered raw materials, charging the powdered rawmaterials into a crucible made of platinum, heating the crucible toapproximately 1500° C. in an electric furnace to melt the raw materials,and pouring the molten glass into a graphite mold, which was pre-heatedto approximately 400° C. After the raw materials were molten completely,the molten material was cooled rapidly, a stress relief was performed byreheating the material to approximately 600° C. and then cooling itgradually, followed by polishing the back side of the obtained glassblock.

The sintered target was obtained by the steps of granulating adesignated amount of powdered raw materials, fabricating the powder intoa fabricated body in a die, and hot pressing the fabricated body at adesignated temperature after dewaxing. The temperature for heattreatment was 900° C. when the cobalt raw material was CO₃O₄, and was1200° C. when the cobalt raw material was CoO.

As a previous step the evaluation of the shape of the disk, a glasssample for a preliminary test in the shape of a thin film as shown inFIG. 2 was prepared, and fundamental material characteristics of theglass thin film were determined. In FIG. 2, the numeral 1 indicates asubstrate, and the numeral 2 represents the glass thin film. In thepresent investigation, a soda-lime glass 0.55 mm thick and 30 mm squarewas used as the substrate 1.

The structure of the prepared film was evaluated by a thin film X-raydiffraction method. Then, it was found that all the prepared films wereamorphous regardless of whether the target was glass or a sintered body,and that glass films were formed.

FIG. 3 indicates the dependence of the transmittance of the glass thinfilms formed using the targets shown in Table 1 on wave length. Thetransmittance was measured using monochromatic light obtained bytreating white light from a light source with a monochromator. Inaccordance with target No. 1, the peak indicating an absorption washardly observed around 300 nm, because of too small a content of CO₃O₄.In accordance with targets No. 2-No. 4, the peak indicating theabsorption by Co could be observed, even though it is small, in theregion of 500 nm-700 nm. No. 3 glass had a transmittance ofapproximately 85% at a wave length of 650 nm.

With the thin films of targets No. 5 and 6, the values of thetransmittance were sufficiently low. However, the transmittance wasdecreased in accordance with decreasing wave length, and it wasindicated that the decrease in the transmittance was caused byscattering. The glass of target No. 7 and the CO₃O₄ of target No. 8 hada sufficiently low transmittance. However, they were reduced in aspattering atmosphere, and a film having a metallic luster was obtained.Therefore, the transmittance was decreased by reflection.

On the other hand, in accordance with the glasses of targets No. 9-No.11, using a raw material of CoO, a peak indicating absorption by Co wasobserved in the vicinity of the region of 500 nm-700 nm. Thetransmittance was decreased in accordance with the increase in Cocontent. The thin film of target No. 11 containing 80% of Co had atransmittance of approximately 5% at wave length of 650 nm. The glass oftarget No. 12 containing CoO of 91.9%, and the film of target No. 13,which was 100% CoO, indicated the same results as target No. 8.

In order to investigate the difference in spectrum of the transmittancecurves in FIG. 3, the valence and oxide conditions of the Co wereanalyzed by XPS. The XPS spectra of Co in the thin films of targets No.3 and 5 are indicated in FIG. 4. In the spectrum of the thin film oftarget No. 3, a peak called a shake up peak exists around 786 eV. Itindicates the presence of a large amount of Co²⁺. On the contrary, theshake up peak can not be observed in the spectrum of the thin film oftarget No. 5. It indicates an oxide condition of CO₃O₄ coexisting withCo³⁺. Accordingly, scattering occurred, and the profile indicated inFIG. 3 was obtained.

The same investigation was performed with other thin films, and it wasfound that, if cobalt existed in the condition of Co²⁺, the spectrumincluded the peak of absorption typical for Co, such as in targets No.2, 3, 10, and 11, and, if Co³⁺ existed, the spectrum became a curveaccompanied with scatter, such as in targets No. 5 and 6.

FIG. 5 indicates the relationship between the plotted transmittance at awave length of 650 nm versus the Co ion content in a target based on thethin film transmittance curves of targets No. 2, 3, 10, and 11. Thetransmittance was decreased in accordance with increasing Coo content,and the transmittance became approximately 30% when the CoO content was60%.

Then, in order to evaluate the Co content in the prepared glass thinfilms, a composition analysis of the film was performed with a secondaryion mass spectrometer (SIMS). A plate cut out from the glass having thesame composition as the target was used as a substrate, and a thin filmhaving the same composition was spattered onto the substrate. Theanalysis was performed from a film forming direction to a depthdirection, so that the compositions of the film and the substrate couldbe evaluated continuously. In the present embodiment, the investigationwas performed using target No. 3 as the target composition.

The results of the analysis are indicated in FIG. 6. It was found thatthe Co content in the thin film was larger than that in the substrate.The Si content in the thin film was smaller then that in the substrate.However, the amounts of change were small, and a large deflection in thecomposition could not be expected. Therefore, the film composition canbe regarded approximately as being the same as that of the targetcomposition.

In accordance with the above investigation, the Co oxide content in theglass thin film is desirably in the range of from 4.5% by weight to 85%by weight as an oxide of CoO, and of from 4.9% by weight to 91% byweight as an oxide of CO₃O₄. If CoO is less than 4.5% by weight, it isdifficult to obtain a sufficient absorption of light. If CoO exceeds 85%by weight, the film bears a metallic luster, and the transmittance isdecreased.

Embodiment 2

Then, the super resolution effect was evaluated by manufacturing ROMdisks, whereon the glass film of the present invention was formed.

FIG. 15 is a block diagram of an example of the optical recordingapparatus used with the optical disk of the present invention. Using theoptical recording apparatus having the above composition, theperformance of the ROM disk of the present invention was evaluated. Thesame apparatus was used on other embodiments.

FIG. 7 indicates a schematic cross section of the manufactured ROM disk.In FIG. 7, the disk includes a polycarbonate substrate 1, a glass thinfilm 2, a SiO₂ protective film 5, and a Al reflector 4, and pits 6represent stored information.

The ROM disk was manufactured by the following steps: First, a pitpastern representing information was formed onto a photoresist by alaser. The pit pattern was duplicated onto a Ni die, and substrates wereformed by injection molding polycarbonate into the Ni die. A glass film160 nm thick was formed onto the substrate by spattering, and after aSiO₂ protective film of 140 nm thick was formed thereon, an aluminumreflecting film 100 nm thick was formed. In the present embodiment, thetarget No. 11 film was formed as the glass thin film. As a comparativeexample, a ROM disk without forming the glass film also wasmanufactured.

The frequency dependency of the regenerating output intensity of themanufactured ROM disk was analyzed with a spectrum analyzer. The resultsare indicated in FIG. 8. The regenerated laser power is 4 mW. It wasrevealed that, in a case when the glass thin film of target No. 11 wasformed, the output level was high until frequency components becamehigher than a case when the glass thin film was not formed. Since thehigh frequency components of a signal are written with a finer pitpattern on the ROM disk, the above result indicated that the output wasregenerated by reading out a finer pit pattern when the glass film wasformed. Therefore, it was found that, when the glass film was formed,the super resolution effect had been obtained.

The same investigations as the above were performed on other glass filmsin Table 1, and the same super resolution effect was confirmed on theglass films of targets No. 3-6, and No. 9-11.

Then, a RAM disk, wherein the glass thin films investigated above wereformed on the substrate, was manufactured, and its characteristics wereevaluated. A schematic cross section of the RAM manufactured inaccordance with the present invention is indicated in FIG. 1. In FIG. 1,the disk includes a polycarbonate substrate 1, a glass super resolutionfilm 2, a recording film 3, a reflecting film 4, and protective films 5,5′. In accordance with the present invention, a circular plate 0.6 mmthick and 120 mm in diameter was used as the polycarbonate substrate 1.A glass film 300 nm thick was formed thereon by a spattering method toform the super resolution film 2. After forming a ZnS—SiO₂ protectivefilm 80 nm thick thereon, a Ge—Sb—Te group phase changing filmrepresenting the recording film was formed approximately 20 nm thickthereon by the same spattering method. Then, after forming a protectivefilm approximately 90 nm thick, an AlTi reflecting film 200 nm thick wasformed thereon.

The glass thin film was formed by the following steps. That is, a glassblock or a sintered body 5 mm in thickness and 120 mm in diameter wasmanufactured as a target, and a backing plate made of copper was adheredonto the back side of the target with an organic adhesive agent forvacuum. Spattering was performed using Argon gas. The power was 200 mW.The film was formed uniformly by rotating the substrate during thespattering. In the present embodiment, the sample target No. 11 was usedas the glass film. As a comparative example, a RAM disk, whereon thegrass film was not formed, was manufactured.

FIG. 9 indicates a relationship of recording mark length versusregenerating output intensity of the RAM disk, whereon recording marksof the same shape were formed with an equal interval. The laser powerfor reading out was 2 mW. In accordance with FIG. 9, it was revealedthat the present embodiment, whereon the glass film of target No. 11 wasformed, had higher regenerating outputs than the comparative example,which did not have the glass film, in the shorter mark length region.Therefore, it was revealed that regeneration was possible to the shortermark length when the glass film is formed. Accordingly, the superresolution effect could tee confirmed with the RAM disk.

The same results as the case of the RAM disk were obtained when all theglass films shown in Table 1 were investigated.

Then, a spatial intensity distribution of the reflecting light in thecases when the above super resolution effect was obtained wereinvestigated. FIG. 14 indicates schematically the intensity distributionof laser light in the laser beam forwarding direction both in the casewhen the glass film was formed and the super resolution effect wasobtained, and in the case when the glass film was not formed. Inaccordance with FIG. 14, it was revealed that the spatial intensitydistribution was approximately a Gaussian distribution in the case whenthe grass film was not formed, but the distribution of the beam wasdeflected toward the laser beam forwarding direction when the glass filmwas formed.

Simultaneously, it was revealed that the beam diameter Q′ at the beamintensity necessary for reading out became smaller in comparison withthe case when the glass film was not formed.

In accordance with the above results, it was revealed that the intensityand the intensity distribution of the reading out light could be variedby using the grass film such as provided in the present embodiment.Furthermore, it was revealed that the super resolution effect could beobtained in the above case.

Embodiment 3

Next, deterioration of the film by repeated regeneration was evaluated.The evaluation was performed by repeatedly irradiating the manufacturedRAM disk with a regeneration signal light and detecting the regeneratedoutput. The pit pitch was 0.3 gm. The glass thin film of target No. 11was used. As a comparative example, a phthalocyanine group organic thinfilm was selected, and the same evaluation was performed.

FIG. 10 indicates a relationship between the output versus the repeatednumber of operations. In accordance with FIG. 10, it was revealed thatthe output of the disk formed with the organic group thin film wasdecreased gradually over the repeated regenerations approximately 10,000times. On the contrary, the output of the disk formed with the glassthin film of the present invention was hardly decreased by repeating theregeneration over 10,000 times. As explained above, it was revealed thatthe optical disk of the present invention maintained the superresolution effect even after repeated regeneration.

The high stability against repeated regeneration could be obtained whenthe glass thin film, with which the super resolution effect was obtainedin the above embodiment 2, among other glass films in Table 1, was usedas the glass thin film.

Embodiment 4

Then, the composition of the glass thin film was investigated. First,paying attention to the content of cobalt oxides in the glass film, arelationship between the cobalt content and the output power wasinvestigated by manufacturing the same RAM disk as the disk in theembodiment 2. The mark length was 0.3 μm. The laser power was 2 mw. FIG.11 indicates a relationship between the cobalt consent and the output.The output power was increased in accordance with an increasing cobaltcontent, and it was found that a high output could be obtained even witha small mark length. In other words, it was found that the superresolution effect could be increased in accordance with an increasingcobalt content. Furthermore, it was found that the output exceeded 5 dBwhen the cobalt content was equal to or more than 20%, in which case itwas possible to beat the output as a signal. However, when the cobaltcontent was less than 20%, the output was less than 5 dB, and it wasimpossible to beat the output as a signal.

The ROM disk shown in FIG. 7 was manufactured, and the output to thehigh frequency component was evaluated by a spectrum analyzer. Then, itwas revealed that the high frequency component could be read out whenthe cobalt content was equal to or more than 20%, but when the cobaltcontent was less than 20%, any significant effect of adding cobalt couldnot be observed.

In accordance with the above investigation, the cobalt content desirablyshould be equal to or more than 20% by weight in any ease of both a ROMand a RAM. In accordance with the investigation in the embodiment 1, thecobalt content desirably should be equal to or less than 91% by weight.

Furthermore, chemical elements to be contained in the glass film wereinvestigated. The mother glass was soda lime glass. The glass containingan oxide of at least one element selected from the group consisting ofTi, V, Cr, Mn, Fe, Co, Ni, and Cu among transition metallic elements,and Nd, Ce, Pr, Sm, Eu, Tb, Ho, Er, and Tm among rare earth elements hadan absorbing spectrum typical of the respective element, and the samesuper resolution effect as the embodiment 2 could be obtained by using alaser beam having a wavelength band capable of absorption.

In accordance with the above results, an optical disk having the superresolution effect could be obtained by using the glass thin filmcontaining at least one element selected from the group consisting ofTi, V, Cr, Mn, Fe, Co, Ni, Cu, Nd, Ce, Pr, Sm, Eu, Tb, Ho, Er, and Tmamong transition metallic elements and rare earth elements.

Next, the composition of the mother glass was investigated. In the aboveembodiments, soda lime glass was used as the mother glass. However, thesame effect could be obtained by using borosilicate glass containingboron. However, when the content of SiO₂ was less than 6% by weight, thestability as glass was low, and crystallization and the like could occurwhen containing an oxide of the transition element or the rare earthelement. When the content of SiO₂ exceeded 80% by weight, the aboveoxide could be hardly included into the glass structure, and it wasdifficult to obtain a stable glass. In accordance with the aboveresults, the content of SiO₂ desirably should be in the range from 6% byweight to 80% by weight.

When the content of alkaline oxide in the glass exceeded 20% by weight,the durability of the glass decreased, and obtaining a stable glass wasdifficult. Accordingly, the content of the alkaline oxide desirablyshould be equal to or less than 20% by weight.

Furthermore, when the consent of boron oxide in the glass exceeded 30%by weight, the oxide of the transition metal or the rare earth elementwas hardly included in the glass structure, and obtaining a stable glasswas difficult. Therefore, the content of boron oxide desirably should beequal to or less than 30% by weight.

In addition to the above indispensable components, an oxide of alkalineearth elements, alumina, zirconia, and the like are desirably containedin the glass as a glass stabilizing agent.

Embodiment 5

Next, the super resolution effect was investigated by manufacturingglass substrates containing a transition metallic element. FIG. 12indicates schematically a cross section of a manufactured RAM disk. InFIG. 12, the disk includes a glass substrate 12, a recording film 3, areflecting film 4, and protective films 5, 5′. The thickness of thesubstrate was 0.6 mm, and a track was formed onto the surface of thesubstrate by reactive ion etching using a photoresist as a mask. FIG. 13indicates schematically the cross section of the ROM disk manufacturedusing the same substrate. In FIG. 13, the disk includes a glasssubstrate 12, a reflecting film 4, and a recording mark 6 representingwritten information. In the present embodiment, soda lime glass was usedas the mother glass, and CoO was contained therein as the transitionmetallic oxide. The super resolution effect was investigated using thesame evaluating method as the embodiment 2 by varying the consent of CoOin the range of 0.01-30% by weight.

The composition of the manufactured glass substrate, the evaluatedresults in vitrification and the super resolution are indicated in Table2. In the evaluated results of the vitrification, the case when glasswas formed without causing crystallization was indicated with O, and thecase when crystallization or devitrification was caused was indicatedwith X. The-evaluated results of the super resolution effect wereindicated by the mark length of 0.3 μm and the output at the spacelength. The reading out laser wavelength was 650 nm.

TABLE 2 S.R.E. Composition (% by weight) Vitri- (output/ No. SiO₂ Na₂OCaO MgO Al₂O₃ CoO fication dB)*¹ 14 72.5 14.0 8.0 4.1 1.4 0.01 ◯  5 1572.5 14.0 8.0 4.1 1.4 0.05 ◯  5 16 72.5 14.0 8.0 4.1 1.4 0.10 ◯ 10 1771.8 13.9 7.9 4.0 1.4 1.00 ◯ 16 18 65.9 12.7 7.3 3.7 1.3 9.1 ◯ 33 1951.8 10.0 5.7 2.9 1.0 28.6 ◯ 41 20 50.7 9.8 5.6 2.9 1.0 30.0 X — 21 66 9B₂O₃9 1 5 10.0 ◯ 33 22 49 9 26 1 5 10 ◯ 35 23 43 9 32 1 5 10 X —Remarks: *¹ Super Resolution Effect

In accordance with the specimens No. 14-19, and 21-23, nocrystallization nor devitrification were observed, and stable glassescould be manufactured. The glass of specimen No. 20 caused a phaseseparation after pouring, and so a stable glass could not be obtained.

In view of the above result, the content of Coo in the glass desirablyshould be equal to or less than 29%.

The glasses of specimen No. 21-23 were the same as the glasses ofspecimen No. 14-19, except for replacing their CaO with B₂O₃.

Regarding the regenerating output, outputs not less than 10 dB could beobtained when the cobalt content was equal to or more than 0.10% byweight, and it was possible to read them out as signals. on thecontrary, when the cobalt content was equal to or less than 0.05% byweight, the outputs were as small as less than 5 dB, and it wasimpossible to read them out.

In accordance with the above results, the content of cobalt desirablyshould be in the range of 0.10-29% by weight. The above effects weresimilar with the glasses which contained B₂O₃ instead of CaO.

In accordance with the present invention, an information recording diskhaving a large capacity, and a small deterioration against repeatedreading out and writing in operations, can be provided. The presentinvention can provide an optical disk having a large capacity whenmanufacturing it with a conventional optical disk manufacturing process.

What is claimed is:
 1. A glass comprising SiO2 and cobalt oxide, whereinsaid cobalt oxide is 4.5-85 wt % as an oxide of CoO or 4.9-91 wt % as anoxide of CO₃O₄.
 2. A glass according to claim 1, further comprising anyone of a transition metal element and a rare earth element.
 3. A glassaccording to claim 2, wherein said transition metal element is at leastone element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co,Ni and Cu, and said rare earth element is at least one element selectedfrom the group consisting of Nd, Ce, Pr, Sm, Eu, Tb, Ho, Er and Tm.
 4. Aglass according to claim 1, further comprising alkaline oxide or B₂O₃,wherein the SiO₂ is 6-8 wt %, the alkaline oxide is equal to or lessthan 20 wt %, and the boron oxide is equal to or less than 30 wt %.
 5. Aglass according to claim 1, wherein the glass is in a shape of a film.6. A glass having a super resolution effect, said glass being amorphousand containing Sio₂ and Co.
 7. A glass according to claim 6, said glassbeing in a shape of a film.
 8. A glass according to claim 7, wherein theglass, in the shape of the film, is provided between a substrate and arecording film, the glass being a glass super resolution film of anoptical information recording medium having said recording film.